I was able to start growing a beard when I turned 14. It grew so quickly after the first six months that I had to give up on being clean shaven 24/7, and decided to just trim it down, instead of just cutting it all off. Every single day, people would say to me: "dude, you look so mature; you look like you're in your 20's." Yet at the same time they said I looked "older-younger", and now I think I understand what that means due to this thread.
Also, my boyfriend, who is ten years my senior, said the same thing when we first met. In the end, I am pretty happy.